fix(coding-agent): exit on lost terminal

Fixes #4144.

packages/coding-agent/src/modes/interactive/interactive-mode.ts

@@ -163,6 +163,16 @@ type CompactionQueuedMessage = {
 	mode: "steer" | "followUp";
 };
 
+const DEAD_TERMINAL_ERROR_CODES = new Set(["EIO", "EPIPE", "ENOTCONN"]);
+
+function isDeadTerminalError(error: unknown): boolean {
+	if (!error || typeof error !== "object" || !("code" in error)) {
+		return false;
+	}
+	const code = (error as NodeJS.ErrnoException).code;
+	return code !== undefined && DEAD_TERMINAL_ERROR_CODES.has(code);
+}
+
 const ANTHROPIC_SUBSCRIPTION_AUTH_WARNING =
 	"Anthropic subscription auth is active. Third-party harness usage draws from extra usage and is billed per token, not your Claude plan limits. Manage extra usage at https://claude.ai/settings/usage.";
 
@@ -3223,6 +3233,15 @@ export class InteractiveMode {
 		process.exit(0);
 	}
 
+	private emergencyTerminalExit(): never {
+		this.isShuttingDown = true;
+		this.unregisterSignalHandlers();
+		killTrackedDetachedChildren();
+		// The terminal is gone. Do not run normal shutdown because TUI and
+		// extension cleanup can write restore sequences and re-trigger EIO.
+		process.exit(129);
+	}
+
 	/**
 	 * Check if shutdown was requested and perform shutdown if so.
 	 */
@@ -3241,12 +3260,26 @@ export class InteractiveMode {
 
 		for (const signal of signals) {
 			const handler = () => {
+				if (signal === "SIGHUP") {
+					this.emergencyTerminalExit();
+				}
 				killTrackedDetachedChildren();
 				void this.shutdown();
 			};
-			process.on(signal, handler);
+			process.prependListener(signal, handler);
 			this.signalCleanupHandlers.push(() => process.off(signal, handler));
 		}
+
+		const terminalErrorHandler = (error: Error) => {
+			if (isDeadTerminalError(error)) {
+				this.emergencyTerminalExit();
+			}
+			throw error;
+		};
+		process.stdout.on("error", terminalErrorHandler);
+		process.stderr.on("error", terminalErrorHandler);
+		this.signalCleanupHandlers.push(() => process.stdout.off("error", terminalErrorHandler));
+		this.signalCleanupHandlers.push(() => process.stderr.off("error", terminalErrorHandler));
 	}
 
 	private unregisterSignalHandlers(): void {